Magnetic devices, such as magnetic random access memories (MRAM), commonly employ a plurality of magnetic tunnel junction (MTJ) devices to store information, each MTJ device comprising one or more magnetic films, or layers. Typically, one of these magnetic layers is a pinned magnetic layer and another of these magnetic layers is a free magnetic layer.
Information is stored in such MTJs as an orientation of the magnetization of the free magnetic layer as compared to an orientation of the magnetization of the pinned magnetic layer. Namely, whereas the pinned magnetic layer has an orientation of magnetization that is fixed, e.g., by a variety of techniques, the free magnetic layer has an orientation of magnetization that is programmed to be either parallel or anti-parallel to that of the pinned magnetic layer, e.g., during a “WRITE” operation of the device.
The free magnetic layer and the pinned magnetic layer typically have a tunnel barrier therebetween. The resistance of this tunnel barrier depends on the orientation of the magnetization of the free magnetic layer relative to the orientation of the magnetization of the pinned magnetic layer. Namely, the resistance of the magnetic tunnel junction is higher when the free magnetic layer and the pinned magnetic layer have orientations of magnetizations that are anti-parallel, as compared to when they are parallel. If the anti-parallel and parallel magnetization states of the magnetic layers are coded to represent binary bits, the state of the magnetic tunnel junction, e.g., either a logic “1” or a “0,” is detected in the “READ” operation by measuring the resistance of the MTJ.
The orientation of the magnetization of a given layer (pinned or free) may be represented by an arrow which, by way of example only, can in some configurations be represented as pointing either to the left or to the right. When the MTJ is sitting in a zero applied magnetic field, the magnetization of the MTJ is stable, pointing either left or right. The application of a magnetic field, however, can toggle the magnetization of the free layer from left to right, and vice versa, to write information to the MTJ. One of the important requirements for data storage is that the magnetization of the MTJ not change orientation unintentionally during the writing process or when there is a zero applied field, or only a small applied field.
However, in many conventional MTJs, the free magnetic layer is typically composed of a single magnetic layer, which can possess a net magnetic dipole moment. The net magnetic dipole moment can undesirably cause a dipole field outside of the MTJ that interferes with the write operation of neighboring MTJs. In addition, this net external dipole field couples strongly to applied fields used to “WRITE” neighboring MTJs and can cause additional MTJs to switch undesirably.
For example, U.S. Patent Application Number US2003/0161180 by Bloomquist et al., entitled “Shared Bit Lines In Stacked MRAM Arrays,” (hereinafter “Bloomquist”), the disclosure of which is incorporated by reference herein, discloses cross-point arrays which include stacked magnetic storage elements, each element comprising a single-layer free magnetic layer. Such cross-point arrays have several notable disadvantages. As mentioned above, a dipole field may undesirably be generated outside of a given MTJ that interferes with the write operations of neighboring MTJs. As such, device density must be greatly reduced, by increasing the separation between adjacent devices, to compensate for this effect. Further, cross-point arrays undesirably generate parasitic current paths, which are competing paths for each given magnetic toggling device. These parasitic paths reduce signal strength during the READ operation. A reduced signal requires that signal averaging be done to read the logical states of the memory cells. As such, inaccuracies and loss of speed surely result.
Therefore, techniques for increasing the density of MTJs in magnetic devices while at the same time decreasing error rates, e.g., in the WRITE operation, would be desirable.